Data communications by wireless LAN (WLAN) typically according to the IEEE802.11 standards are now widely used. They are used in personal computers (PCs); peripherals such as printers, hard disk drives, broadband rooters, etc.; electronic appliances such as facsimiles, refrigerators, standard-definition televisions (SDTVs), high-definition televisions (HDTVs), digital cameras, digital video recorders, cell phones, etc.; and as signal-transmitting means in place of wires in automobiles and aircrafts, and wireless data transmission is conducted among these electronic or electric appliances.
There are now pluralities of standards of wireless LAN. For instance, IEEE802.11a is adapted to high-speed data communications of 54 Mbps at maximum in a frequency band of 5 GHz, using an OFDM (orthogonal frequency division multiples) modulation system. Incidentally, there is IEEE802.11h as the standard for making IEEE802.11a usable in Europe.
IEEE802.11b is adapted to high-speed communications of 5.5 Mbps and 11 Mbps in an industrial, scientific and medical (ISM) band of 2.4 GHz that can be freely used without wireless license, using a direct sequence spread spectrum (DSSS) system.
IEEE802.11g is adapted to high-speed data communications of 54 Mbps at maximum in a 2.4-GHz band like IEEE802.11b, using the OFDM (orthogonal frequency division multiples) modulation system.
Explanation will be made below using IEEE802.11a and IEEE802.11h as a first communication system, and IEEE802.11b and IEEE802.11g as a second communication system, if necessary.
A multi-band communication apparatus using such WLAN is described in JP2003-169008A. This multi-band communication apparatus comprises two dual-band antennas capable of transmitting and receiving in two communication systems having different communication frequency bands (IEEE802.11a, IEEE802.11b), two transmitting/receiving means for modulating transmission data and demodulating receiving data in each communication system, pluralities of switch means for connecting the antennas to the transmitting/receiving means, and switch control means for controlling the switch means, so that it can perform diversity receiving (see FIG. 33).
As another example, JP2002-033714A describes a multi-band communication apparatus using one multi-band antenna. This multi-band communication apparatus comprises a 2.4-GHz-band, front-end circuit and a 5-GHz-band, front-end circuit each comprising a switch circuit, amplifiers, a mixer, etc., a switch circuit SW1 for selectively connecting one of them to a common multi-band antenna, switch circuits SW2, SW3 for switching transmitting/receiving circuits, and a switch circuit SW4 connected to an intermediate-frequency filter BPF (see FIG. 34).
In multi-band communication apparatuses for WLAN, a carrier sense multiple access (CSMA:) system is adopted to scan the frequency to sense the receivable frequency channel (carrier) before starting communications.
In the multi-band communication apparatus of JP2003-169008A, to conduct this scanning operation, the antenna ANT1 is connected to a receiving terminal Rx of the transmitting/receiving means of 802.11a, and the antenna ANT2 is connected to a receiving terminal Rx of the transmitting/receiving means of 802.11b, by six single-pole, double-throw (SPDT) switch means (SW1-SW6). The transmitting/receiving means of 802.11a is scanned in a 5-GHz band, and the transmitting/receiving means of 802.11b is scanned in a 2.4-GHz band, to sense all receivable vacant channels.
The next step is to compare a receiving signal in a 5-GHz band received by the dual-band antenna ANT1 with a receiving signal in a 2.4-GHz band received by the dual-band antenna ANT2, and select one of the two communication systems by which a more desired signal is received, as an active communication system.
After this scanning operation, the other antenna is connected to the transmitting/receiving means of the selected communication system to perform receiving without changing the receiving channel. The receiving signals from two antennas are compared, and an antenna capable of performing better receiving is selected as an active antenna to conduct diversity receiving.
Also, in the multi-band communication apparatus of JP2002-033714A, receiving signals in 2.4-GHz and 5-GHz bands are scanned by one dual-band antenna ANT1 to sense all receivable vacant channels, and a channel of a desired communication system is selected.
As in the conventional multi-band communication apparatuses, the selection of communication system channels based on the comparison results of receiving signals obtained from different antennas or one antenna connected to each communication system is under unnegligible influence by disturbances such as noises from other communication systems and electronic equipments, phasing, etc., resulting in the likelihood that a communication system channel to be selected is erroneously sensed as busy, and that a communication system in which a receiving signal has the largest amplitude is not selected.
There are electromagnetic waves leaking from electronic ovens, noises from communication systems such as Bluetooth®, radio frequency identification (RFID), etc. particularly in a frequency band of 2.4 GHz, and there are communication systems of cell phones having relatively large transmission power, such as WCDMA (wide band code division multiple access), etc., in nearby frequency bands. These high-frequency signals interfere communications in the WLAN systems. However, the conventional multi-band communication apparatuses do not have any measure to cope with such noises, etc.
Also, because high-frequency signal paths should be switched by many switch means in the conventional multi-band communication apparatus, control becomes complicated depending on the number of switch means. Because the switch means have transmission loss to some extent, many switch means disposed in paths from antennas to transmitting/receiving means increase transmission loss according to their number. Particularly at the time of receiving, the quality of a high-frequency signal input through an antenna is disadvantageously deteriorated. In addition, because power consumed by switching means is not negligible in battery-driven equipments such as note PCs, cell phones, etc., it has been requested to constitute a multi-band high-frequency circuit with few numbers of switch means.
In IEEE802.11h, the function of dynamic frequency selection (DFS) or transmission power control (TPC) is newly required. The TPC function is to reduce power consumed for transmission to a minimum level when good communications can be made even at a suppressed transmission power, for instance, in a case where mobile terminals are close to a base.
Various circuit elements such as switch circuits, etc. disposed between the output port of a power amplifier and a multi-band antenna generate insertion loss. Because this insertion loss has frequency characteristics, output power from a multi-band antenna is not constant depending on frequency channels used by the multi-band high-frequency circuit, but variable depending on the channels used. Accordingly, it is necessary to precisely control output power from the amplifier.
For instance, to perform a TPC function in the multi-band communication apparatus of JP2003-169008A, a coupling circuit (for instance, a directional coupler) should be connected between the transmitting/receiving means of IEEE802.11a and the switch circuit SW3, and between the transmitting/receiving means of IEEE802.11b and the switch circuit SW4, to input a detection signal from the directional coupler to a detection circuit, thereby controlling an output signal based on the resultant detection voltage. However, this method needs directional couplers, detection diodes and smoothing circuits in both 2.4-GHz band and 5-GHz band. Further, when a common control circuit is used, an analog switch for selecting detection voltage terminals in a 2.4-GHz band and a 5-GHz band is also needed. This increases the number of parts, resulting in difficulty in the miniaturization of communication apparatuses.
High-frequency circuits for WLAN also need filter circuits for removing unnecessary frequency components contained in transmission signals and receiving signals, in addition to switch circuits for switching diversity switches, transmitting circuits and receiving circuits. Further, balanced-to-unbalanced converters for converting balanced signals to unbalanced signals, and impedance-converting circuits are needed.
When contained in cell phones or note PCs, or used as network cards of PCMCIA (personal computer memory card international association), it is strongly desired to miniaturize multi-band communication apparatuses.